Because the best backup for GPS isn't always another GPS

Starting today and running through to the 22nd of February, the US Department of Defense will be running tests on the GPS system which will render the signal unreliable or unavailable for portions of the southeast Atlantic seaboard. 

In unrelated  news, the Starpath School of Navigation Celestial Navigation series begins on February 22nd, course details and registration are available here.

SOUTHEAST ATLANTIC COAST: GPS Testing Information

THE GPS NAVIGATION SIGNALS MAY BE UNRELIABLE FROM 20 JAN 2011 - 22 FEB 2011 FROM 0000Z - 0245Z DUE TO TESTING ON GPS
FREQUENCIES USED IN SHIPBOARD NAVIGATION AND HANDHELD SYSTEMS. GPS SYSTEMS THAT RELY ON GPS, SUCH AS E-911, AIS AND DSC,
MAY BE AFFECTED WITHIN A 150 NM RADIUS OF POSITION 30 49.09N 80 28.18W. DURING THIS PERIOD GPS USERS ARE ENCOURAGED TO
REPORT ANY GPS SERVICE OUTAGES THAT THEY MAY EXPERIENCE DURING THIS TESTING VIA THE NAVIGATION INFORMATION SERVICE (NIS)
BY CALLING (703) 313-5900 OR BY USING THE NAVCEN WEB SITE'S GPS REPORT A PROBLEM WORKSHEET AT WWW.NAVCEN.USCG.GOV.

(USCG Local Notice to Mariners 02/2011)